1. Field of the Invention
The present invention generally relates to an image forming apparatus, and a threshold setting method.
2. Description of the Related Art
As a printing system for forming images on both sides of a web without a feed hole as typified by a continuous long belt-like sheet, for example, as described in Japanese Patent No. 3680989, a printing system capable of printing an image on the second side of a web correctly so as to align with an image on the first side even if the web discharged from a first printing device is shrunk or expanded with environment has been proposed and put to practical use.
FIG. 1 shows a total configuration of an electrophotographic device applied to a conventional printing system. In FIG. 1, “W” denotes a web; usually, it is a paper web in most cases. The web W delivered from a feeding device (not shown) is fed toward a web buffer mechanism 2 by being guided by a guide roller 1 arranged on a feed path to thread through a printing device P. Then, the web W passes through a guide member 3, a foreign-body removing mechanism 4, a tension applying mechanism 5, a guide shaft 6, and a guide plate 7, and is fed into an imaging unit 10 by feed rollers 8 and 9. In the imaging unit 10, through charging, exposure, and developing processes, a toner image is formed on a photosensitive drum 101, and after that, the toner image is transferred onto the web W by the action of a transfer unit 105.
The web W gets off a feed belt 11, and is fed to a fixing unit 13 via a buffer plate 12. When reaching the fixing unit 13, the web W is preheated by a preheater 13a. After that, the web W is fed while being sandwiched in a nip between a pair of fixing rollers composed of a heat roller 13b and a pressure roller 13c, and applied with heat and pressure by the heat roller 13b and the pressure roller 13c thereby fusing and fixing the toner image on the web W.
Furthermore, a reference numeral 16 denotes a mark detecting unit (a mark sensor) for detecting a positioning mark 17 formed on the web W as shown in FIG. 2. The mark sensor 16 is an optical sensor composed of a light-emitting element and a light-sensitive element. The positioning mark 17 is formed on near the top of each page, and is used as a reference position for aligning positions of images that are formed on both sides of the web W by first and second printing devices which are connected to each other. Specifically, the second printing device detects the positioning mark 17 formed by the first printing device, and prints out an image on the second side of the web W correctly so as to align with an image on the first side by controlling, i.e., changing the rotating speeds of the feed rollers 8 and 9 and the photosensitive drum 101 (for example, see Japanese Patent No. 3680989 and Japanese Patent Application Laid-open No. 2003-266825).
Conventionally, in such a printing system, when an image is printed to fit in a business form of a preprinted sheet, the first printing device needs to form the positioning mark 17 on the preprinted sheet. However, to bother to form the positioning mark 17 on the preprinted sheet in addition to the business form causes an extra cost; therefore, recently, a method to use a portion of the business form, such as a company name or logo printed at the specified position on each page, as a positioning mark has been implemented.
A company name or logo on a preprinted sheet differs in layout or a color from one business form to another, so it is necessary to provide a way to detect a plurality of colors at an arbitrary position; therefore, by providing a mechanism capable of moving the mark detecting unit to an arbitrary position in a main-scanning direction or by making a sensitivity adjustment in accordance with a color of the company name or logo using an optical sensor with sensitivity to the plurality of colors, the company name or logo can be used as a positioning mark.
On the other hand, as a method for the sensitivity adjustment, there is a method to cause the mark sensor 16 to recognize a color of the positioning mark 17 and a ground color of the web W and set an intermediate color of the two colors as a threshold. At this time, it is preferable to arrange the positioning mark 17 in the center of a detectable area of the mark sensor 16. As a method to detect the center of the positioning mark 17, for example, as proposed in Japanese Patent Application Laid-open No. 2002-207338 and Japanese Patent Application Laid-open No. 2002-174936, there is a detection method in which using a sensor that outputs 0 V when the mark sensor 16 detects the positioning mark 17 and 5 V when the mark sensor 16 does not detect the positioning mark 17, the position of the center of a line connecting the center voltage in a fall region of an analog signal output from the sensor and the center voltage in a subsequent rise region is set as the center of the positioning mark 17.
Furthermore, Japanese Patent Application Laid-open No. 2000-318221 has proposed a detection method in which using the mark sensor 16 that outputs an analog voltage according to a color of an object to be detected, a voltage between a voltage value±α that is output when detecting the ground color of the web W, and a peak voltage value that is output when detecting the positioning mark 17 is set as a threshold voltage. Two points at which an output voltage of the mark sensor 16 intersects with the threshold voltage near the peak voltage are obtained, and then the midpoint of the two points is set as the center of the positioning mark 17.
Moreover, as a simplified method, there is known a method that a scale 18 as shown in FIG. 8 is provided, and an operator visually sets the positioning mark 17 so that the positioning mark 17 is positioned roughly in the center of a detectable area φ of the mark sensor 16.
The mark sensor 16 is a sensor that the light-sensitive element detects that a light emitted from the light-emitting element is reflected or absorbed on the basis of a color of a target object, and recognizes a color level of the target object according to an amount of light received. Therefore, when the ground color of the web W and the positioning mark 17 are both contained within the detectable area φ, an amount of light received, i.e., a color level varies according to the proportion of the positioning mark 17 in the detectable area φ.
Consequently, when the visual position adjustment is made, even though an operator believes that the positioning mark 17 is positioned in the center of the detectable area φ of the mark sensor 16, the positioning mark 17 may deviate from the detectable area φ, or the position of the positioning mark 17 varies among operators, and therefore, a result of the sensitivity adjustment varies.
FIG. 9A is a diagram illustrating a case in which the positioning mark 17 is positioned in the center of the detectable area φ of the mark sensor 16; FIG. 9B is a diagram illustrating a case in which the positioning mark 17 is not positioned in the center of the detectable area φ of the mark sensor 16. FIG. 10 is a diagram showing color levels of the ground color of the web W and the positioning mark 17 recognized by the mark sensor 16 and a threshold level in the cases shown in FIGS. 9A and 9B.
With respect to an object of which the size is larger than the detectable area φ like the web W, it is easy to make a position adjustment, so a variation in color recognized by the mark sensor 16 is small. Namely, L0≈L0′ holds. On the other hand, when an object is small in size like the positioning mark 17, it is difficult to arrange the object to fit perfectly in the detectable area φ of the mark sensor 16. When a sensitivity adjustment is made in a state where the positioning mark 17 deviates from the detectable area φ as shown in FIG. 9B, a color level L1′ of the positioning mark 17 is in a relation of L1′>L1. Furthermore, when an exactly intermediate level of the color levels of the ground color of the web W and the positioning mark 17 is set as a threshold, a threshold level TH′ is in a relation of TH′>TH.
In this manner, when a sensitivity adjustment is made in a state where the positioning mark 17 deviates from the detectable area φ, the mark sensor 16 recognizes the color of the positioning mark 17 as a color close to the ground color of the web W, so a contrast with the color of the web W is insufficient; therefore, if a disturbance, such as flapping of the web, is produced while the web is fed, there arises a problem in that the positioning mark 17 is incorrectly detected even though there is no positioning mark 17.
Furthermore, to arrange the positioning mark 17 in the center of the detectable area φ of the mark sensor 16 using any of the above-described methods disclosed in Japanese Patent Application Laid-open No. 2002-207338, Japanese Patent Application Laid-open No. 2002-174936, and Japanese Patent Application Laid-open No. 2000-318221, data processing, such as an analog-to-digital (A/D) conversion of an output signal of the mark sensor 16, is required and a circuit configuration becomes complex.